Rossby Waves

Figure 12.13 shows that the upper westerlies do not flow steadily towards the east, but sway from side to side as Rossby waves, named after the Swedish meteorologist Carl-Gustav Rossby. He explained them in 1939 in terms of the Coriolis effect (Note 12.J). The waves are bends in the path along which the winds blow, most evident in the jet stream. If there are six such waves in circling the globe, we say that the wave number is six. The waves travel against the wind direction, at speeds which depend on their size.

Rossby waves may form on account of horizontal variations of low-atmosphere temperature. As an example, relative warmth over a peninsula in summer leads to vertical expansion of the air, causing a local raising of the level at which pressures are, say, 200 hPa (Note 12.E; Figure 12.14). Thus, there is an increased pressure over the land at any stipulated elevation in the upper atmosphere; we say there is a ridge of high pressure. This promotes a 'thermally direct circulation', like a Hadley cell (Section 12.3) or sea breeze (Chapter 14), as shown in Figure 12.14. The upper-level ridge also affects the flow of upper winds, which flow geostrophically, following the isobars around the region of high pressure, anticlockwise in the southern hemisphere. Conversely, an adjacent cooler surface has the effect of creating a clockwise bend in a jet stream in the southern hemisphere, around a trough of low pressure. So there is a series of alternately left and right loops in the wind's path (Figure 12.15). This is an example of a surface condition affecting upper-level winds. We shall see in the next chapter that the two levels are closely linked, with interactions in both directions.

The initial deflection forming a Rossby wave might have been a coastal difference of temperatures, but the cause could be a mountain range, discussed later. However, it should not be inferred from the fixed positions of coasts and mountains that the Rossby waves are tied to particular locations. There is some tendency that way (which we discuss in connection with 'blocking' in Section 12.5 and Chapter 13), but

Rossby Waves Mountains

Figure 12.13 Trajectory of a balloon released from Christchurch (NZ) and carried along at a level of 12km. The numbers indicate the days elapsed from the time of launch.

Figure 12.13 Trajectory of a balloon released from Christchurch (NZ) and carried along at a level of 12km. The numbers indicate the days elapsed from the time of launch.

the dominant feature of the waves is their mobility.

The sharpness of the turning, i.e. the amount of rotation (or vorticity) created by the ridge or trough, depends on both the strength of the wind through the wave and the length of the wave (Note 12.K). Vorticity is a useful concept because it is a property that tends to be 'conserved', to persist. Thus, whirls that form behind an obstacle in a river and then are carried away by the flow do not disappear as soon as they are shed from the obstacle. The vorticity which is conserved is the absolute vorticity, which has two components from separate sources—the turning of the wind over the Earth, and also the turning of the Earth in space. The former contributes relative vorticity and the latter planetary vorticity. These will be discussed later.

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  • henna
    How rossby waves affect weather?
    8 years ago
  • kacie
    How do rossby waves form?
    8 years ago

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